The overall goal of the proposed research is to investigate the individual etiology of Attention-Deficit/Hyperactivity Disorder (ADHD) and the shared etiologies of ADHD with other disruptive behavior disorders, namely Conduct Disorder (CD), and Oppositional Defiant Disorder (ODD). In addition the proposed research seeks to investigate how these etiologies may differ as a function of age, intelligence (IQ), socioeconomic status (SES), and gender. Twin pairs of children in which at least one twin shows elevated levels of ADHD symptomatology will be included in the study. Parent reports of ADHD, CD, and ODD symptomatology will be obtained as well as estimates of IQ and SES. Univariate and bivariate behavioral genetic analyses will be employed to estimate the genetic, shared environmental, and non-shared environmental contributions to the etiology of ADHD, its overlap with CD and ODD, as well as to test whether these etiologies vary as a function of age, IQ, SES, and gender. Specific knowledge of the etiology of disruptive behavior disorders as a function of age, gender, IQ, and SES is important for understanding how these disorders develop and can potentially be relevant to their prevention and treatment. Moreover, a thorough behavioral genetics analysis of the phenotypes of the disruptive behavior disorders can greatly aid future molecular genetics analyses.